1. Field of the Invention
The present invention relates to an image signal readout method and apparatus, and an image signal readout system for reading out an image signal from an image detector which includes multitudes of pixels, each having a TFT switch, disposed two-dimensionally.
2. Description of the Related Art
Recently, flat panel detectors (FPDs) having a TFT active matrix array on which an X-ray sensitive layer is disposed have been put into practical use. They are capable of directly converting X-ray information to digital data, and have an advantage over the conventional imaging plates in that they allow instantaneous image verification and motion image monitoring, so that they are spreading rapidly.
First, the structure of a conventional radiation image detector will be described with reference to FIG. 7. FIG. 7 is a schematic equivalent circuit of 3×3 pixels. In FIG. 7, the reference numerals 111, 112, 113, and 114 respectively denote a photoelectric conversion element, a TFT switch, a scanning wire, and a data wire.
Radiation, such as X-rays or the like, enters from above the surface of FIG. 7, which is converted to electrical signals by the photoelectric conversion elements 111 and the charges thereof are stored in each of the pixels. Thereafter, the TFT switches 112 are sequentially activated by the scanning wires 113 to transfer the stored charges to the data wires 114 connected to either one of the source/drain electrodes of the TFT switches 112, and the charges are detected by the signal detectors 115.
Such radiation image detector called FPD has characteristic features that accurate radiation images are detected since it is capable of directly detecting image signals. But, there may be cases in which various types of noise are added to the essential image signals to be detected due to various reasons.
For example, one type of noise is caused by leak current of the TFT switch. It is preferable that no leak current flows through the TFT switch that selects a pixel to be detected while it is in OFF state. But, a certain amount of leak current flows due to the characteristics of the device, which is added to the image signals. In order to solve this problem, Japanese Unexamined Patent Publication No. 2003-319264 proposes a method in which the leak current is read out from each of the TFT switches while it is in OFF state, and image signals are corrected using the value of the leak current of each of the TFT switches.
The method proposed in Japanese Unexamined Patent Publication No. 2003-319264, however, does not necessarily improve the quality of an image. Because, the method uses a process for subtracting a leak current component from the image signal in the correction of the amount of leak current. The leak current component is random noise, so that the subtraction of the leak current component from the image signal only results in the addition of the leak current component to the image signal and the amount of noise is increased. Thus, the method has a problem that it may degrade image quality.
In view of the circumstances described above, it is an object of the present invention to provide an image signal readout method and apparatus, and an image signal readout system capable of reading out an image signal from an image detector without causing image quality degradation described above.